To go back to the roads discussion, here is a system that echoes some of
the ideas proposed so far. Roads can be built in levels. Each level of
road reduces the time to travel either N-S _or_ E-W by one day, to a
minimum of 4 days. Each level of roads costs 20,000 effect and 1000 stone
(twice that of a castle). Hence, many levels of road can be build through
a mountain (effectively cutting a tunnel through it) while only a few can
be built on plains (and only a few need be). Every road declines in
effectiveness one level each game year (through wear and tear and
weathering). A road can be attacked just like any other structure to
damage it.

In fact, you could treat a road just like a structure, requiring move
orders to use the road's ID number to gain it's benefits, and allowing a
noble to move into a road to 'own' it and block usage of it unless a toll
is paid (much along the way a ferry operates under the current rules). A
single stretch of road could be considered one structure even though it
runs through many provinces.

This whole idea would probably take a lot of code to implement, but I think
would add quite a bit to the long-term game. Imagine one merchant debating
on whether to pay a toll and make it to the next city in 20 days, or to
avoid the toll, take a longer time to cross the mountains, and maybe risk
getting beat by a competitor.